Communication systems employ coding to ensure reliable communication across noisy communication channels (e.g., satellite communications channels). These communication channels exhibit a fixed capacity that can be expressed in terms of bits per symbol at certain signal to noise ratio (SNR), defining a theoretical upper limit (known as the Shannon limit). The Shannon-Hartley theorem expresses the maximum rate at which information can be transmitted over a communications channel of a specified bandwidth in the presence of noise. As a result, coding design has aimed to achieve rates approaching this Shannon limit. One such class of codes that approach the Shannon limit is Low Density Parity Check (LDPC) codes.
In clear sky scenarios, the data transmission capability of a given satellite terminal depends on the power and antenna gain of the terminal antenna, and the capability of the satellite being used. For a typical small terminal, the data rate is limited by the satellite antenna gain over temperature, G/T. Higher data rates are supported by higher the G/T, and G/T depends on the satellite design, and location of the terminal within the coverage of the satellite. Higher data rates can also be achieved by using a higher power transmitter, or higher gain antenna at the terminal. By definition, however, antenna gain is limited for small antenna. Additionally, because of the wider beam-width of the small antenna, and the tight spacing of the satellites on the geosynchronous satellite orbit, small terminals must transmit their signal within an off-axis spectral density limit to ensure that they do not cause excessive interference to the adjacent satellites.
Further, both the Federal Communications Commission (FCC) and the U.S. and International Telecommunication Union (ITU) have set mask regulations in terms of angle from the antenna beam center. As a result, for a small terminal, the maximum transmit power in a given bandwidth cannot be exceeded due to such regulations. The regulations preclude the option of using higher transmit power to a satellite with lower G/T. For these cases, the only option to achieve reliable communications is to reduce the transmission data rate. When the transmission data rate is reduced, however, the occupied bandwidth of the signal is reduced accordingly. Hence, if a terminal is already using the maximum power allowed by the regulation, then reducing data rate would not provide any benefit.
Therefore, there is a need for an approach for closing communications channel links (e.g., for small terminal applications in satellite communications systems), at lower effective data rates, in a most power efficient manner, while still meeting regulatory requirements.